In a lithographic process, a single particle landing on the front side of a reticle can result in the need of reworking or scrapping several lots of wafers. During the development of DUV lithography, the problems caused by particles landing on a mask or reticle were solved by employing transparent pellicles that maintain such particles in a plane that is out-of focus with respect to the printed image. However, alternatives were sought to the use of pellicles for EUV lithography given that no known material is entirely transparent to EUV light.
The initial EUV tools were built based on ultra-clean vacuum chambers with the expectation that very few particles would be emitted by the internal components of the tool. Nevertheless, as particles are eventually emitted by mechanical components, light sources and wafers, this initial approach to particle control depends on an extensive metrology and reticle cleaning regime in order to minimize the number wafers that need to be reworked/scrapped after a particle eventually lands on the surface of an reticle and starts causing printable defects.
In view of such shortcomings, EUV pellicles comprising membranes as thin as 50 nm have been developed. However, said pellicles are extremely fragile structures that have a limited lifetime, can be a source of particles when broken, can have a non-negligible effect on critical dimension uniformity, complicate defect inspection, imply the loss of at least 20% of the light reaching the wafer and have to be removed for cleaning, patterning and non-actinic inspection.
Given the low throughput of early EUV scanners and the likely Esize values and number of masks needed for advanced manufacturing nodes, there is a need of increasing the power of EUV light sources for lithographic scanners and as well as for actinic inspection tools. However, using an EUV pellicle for particle control in such applications causes a loss of throughput due to light absorption comparable to adding an additional mirror to the projection optics without any increase in resolution. Furthermore, as more powerful EUV sources become available, the lifetime of EUV pellicles could also be reduced due to increased thermal loads and higher acceleration of the reticle stage leading to more frequent disruptions due to broken pellicles.
The instant invention provides a pellicle-less approach to particle control that is based on active and passive particle deflection methods.